Variation in Current Density of Aluminum Scrap-Based Propeller Anodization to Increase Surface Hardness ^†

Abstract

Aluminum has the advantages of being lightweight and rust-resistant, and having high strength and durability. Aluminum scrap is a recycled material and is reused in its production process, for example, for propellers. Because it is used in conditions that require good durability, a coating that can increase the hardness and strength of aluminum is introduced. This study used the anodization method with a H₂SO₄ electrolyte medium and variations in current density of 0.03 A/cm², 0.035 A/cm², and 0.04 A/cm². The anodization time was 45 min. It was found that the hardness of the specimen increased from the initial hardness of 189 HL.

1. Introduction

Aluminum is one of the materials widely used in the maritime industry, especially in the manufacture of ship propellers, because it has the properties of light weight, corrosion resistance, and good thermal conductivity. However, pure aluminum and its alloys have relatively low hardness, making them susceptible to wear and deformation when operating in abrasive marine environments. Therefore, efforts are needed to improve the hardness and surface resistance of aluminum-based propellers in order to extend their service life and reduce maintenance costs.

One of the effective methods for improving the mechanical properties and corrosion resistance of aluminum is the anodizing process. Anodizing is an electrochemical process that forms a hard oxide layer on the aluminum surface, which offers protection against abrasion and corrosion. The main parameters in anodizing, such as current density, electrolyte type, and process time, greatly affect the thickness and characteristics of the oxide layer formed [1,2].

On the other hand, the use of aluminum scrap as a raw material for propellers is an innovative solution in supporting the principles of a circular economy and green technology. The fabrication process of aluminum scrap is also relatively easy. One method can use casting techniques. Recycled aluminum has great potential to be developed as a structural material, but its mechanical properties can vary depending on the recycling process and surface treatment performed. By applying optimal anodizing techniques, the mechanical properties of aluminum scrap can be improved so that it is suitable for use in maritime applications [3].

Unfortunately, the use of aluminum scrap with the casting method still causes many problems. One of them is a decrease in the mechanical properties of cast aluminum scrap products. The most striking thing is that cast products are prone to porosity defects. These pores then cause a decrease in the mechanical properties of the aluminum. The load distribution becomes uneven, and if these pores are on the surface, it is also easier for pitting corrosion to be initiated [4].

This study aimed to analyze the effect of current density variation in the anodizing process on the surface hardness of aluminum scrap-based propellers. By elucidating the relationship between anodizing parameters and the mechanical properties of the material, it was expected to reveal optimal conditions that improve propeller performance in real applications. The results of this study not only contribute to improvements in propeller manufacturing technology, but also support the utilization of recycled materials in sustainability-based industries.

Current affects the thickness of the oxide formed. The higher the current, the greater the growth in oxide thickness and the harder the aluminum. However, if the current density is too high, the hardness will decrease because the oxide layer is more porous, reducing the resistance of the aluminum alloy. This means that, the higher the current, the harder the aluminum but the lower the ability to protect against corrosion layers [5,6]. Current density itself represents how much electric charge is on the surface area. Because the morphology of the pores themselves spreads downwards, using a difference in current density can help the electric charge to travel more into the pores.

In this regard, a deeper understanding of the effect of current density on the hardness of aluminum anodization is important for designing an efficient anodization process and producing an oxide layer with good mechanical properties.

2. Methods

2.1. Materials

In the anodizing process, the material used was aluminum scrap, which acts as the anode (which was to be anodized). For the cathode, carbon was used. The size of the aluminum scrap used for the hardness test was ±2.5 × 2 cm, the specimen was resin, and one surface was left to be anodized. Then, sanding was carried out with a size of 80–2000 to smooth the specimen. Autosol was used to polish the specimen pieces so that the surface was shiny. After that, degreasing was carried out to remove dirt such as fat, oil, dust, and other dirt that stuck to the surface using a 10 vol% NaOH solution for 20 s.

This study investigated the anodizing coating potential of recycled aluminum propellers. The propellers were sectioned into specimens with dimensions of 4 cm × 6 cm, preserving the original surface slope and curvature to maintain geometric fidelity. Prior to the anodizing process, chemical composition analysis was performed using X-ray fluorescence (XRF) in accordance with ASTM E34 standards on specimens without anodizing treatment. This characterization aimed to identify the aluminum alloy type resulting from the aluminum scrap casting process.

The anodizing process was subsequently conducted on the prepared specimens. The experimental setup included a DC power supply as the electrical source, carbon electrodes serving as counter-electrodes, and thermocouples employed to monitor and control the electrolyte temperature during the anodizing process.

The next step was etching, with surface erosion using a 30 vol% nitric acid solution (HNO₃) for 1 min. After etching, the specimen was rinsed using distilled water and left to dry, so that it was ready for the next treatment process (anodization). Next, the anodization process was carried out with sulfuric acid electrolyte (H₂SO₄). At this stage, the cathode began to electrochemically coat the anode so that a thick and strong layer was formed, which could provide good corrosion protection.

2.2. Anodizing Setup

In this study, there were 4 specimens; the experimental variables were designed by making variations in current density at normal room temperature and with the same duration of 45 min. The electric current to be used depended on the size of the surface area to be anodized by referring to the following equation, where i is the electric current (A), J is the current density (A/cm²) and A is the surface area (cm²). The variations in current density used can be seen in

Table 1. Parameter setup for anodizing.

Specimen	Current Density (A/cm2)	Code
1	As received	A
2	0.030	B
3	0.035	C
4	0.040	D

below.

$$i=J\times A$$

(1)

After the anodizing process, the specimen was rinsed and dried. Then, the next stage was entered, namely hardness testing; this test was carried out to determine the hardness value of the layer using a pen type Leeb hardness tester, which can measure the hardness of a material by determining the ratio of the rebound velocity to the impact velocity.

3. Results and Discussion

3.1. Chemical Composition

Table 2. Percentage chemical composition.

No	Element	Composition
1	Al	92.91
2	Si	3.87
3	Zn	0.96
4	Fe	0.97
5	Cu	0.88
6	Pb	0.41

presents the elemental chemical composition data of the specimen. Based on the results of the chemical composition analysis, the material used in this study was classified as an Al-Si aluminum alloy, characterized by a silicon (Si) content of 3.87%.

Al-Si alloys possess several advantageous properties, including excellent castability, good weldability, superior corrosion resistance, lightweight characteristics, a low coefficient of thermal expansion, and high electrical conductivity. Due to these notable attributes, Al-Si alloys are widely utilized in various industrial applications [7]. One type of Al-Si alloy is widely used for automotive and industrial parts, and its use as a ship propeller remains appropriate. Its mechanical properties can be effectively modified through sand casting. However, high levels of iron (Fe) in the alloy are often undesirable, as Fe is considered an impurity that reduces the strength of Al-Si alloys. Reducing the amount of Fe significantly improves the strength and ductility of aluminum alloys [8].

Al-Si alloys with high Si content also allow the formation of Mg₂Si intermetallic phases. This phase has better corrosion resistance. The Mg₂Si phase will form MgO and SiO₂ when exposed to oxidizing ions. This oxide compound acts as a passive layer to protect the surface of the aluminum alloy [9].

3.2. Hardness Test

From the results of the anodization process that has been carried out, the effect of current density during the anodization process on the hardness of aluminum scrap is shown in

Table 3. Hardness test of anodized propeller aluminum scrap.

Code	Hardness (HL)			Average Hardness (HL)
	1	2	3
A	183	212	173	189
B	173	185	214	191
C	212	218	211	214
D	186	240	228	218

.

After conducting hardness testing on sample (A), a hardness value of 191 HL was obtained; for sample (B), a hardness value of 214 HL was obtained; for sample (C), a hardness value of 218 HL was obtained; and for sample (D) without anodization, a hardness value of 189 HL was obtained. Based on the image above, the results of the hardness test show that sample (C), with a hardness value of 218 HL, has the highest hardness value. This Figure 1 shows that the anodizing current density affects the hardness value of the material. However, if the current density is too high, the formation of pores will be faster such that it has the potential to reduce its hardness [10,11].

4. Conclusions

Based on the data analysis and discussion of the results of this study, several important things can be concluded as follows:

• The anodizing process can increase the hardness of the surface layer of scrap aluminum material.
• Current density affects the hardness results for aluminum scrap. The greater the current density used, the greater the hardness value of the aluminum scrap.
• The hardness value produced on the surface of aluminum scrap (C) with a current density of 0.04 A/cm² was the highest, namely 218 HL.